1. Field of the Invention
This invention relates to polymer film-metal composites useful for making printed circuits which are resistant to delamination, particularly when exposed to high temperatures and/or humidity.
2. Description of Prior Art
Traditionally, flexible printed circuits have been used in lieu of discrete wiring harnesses to interconnect components in electronic equipment applications where three-dimensional packaging efficiency, reduced weight, and long-term flexural endurance are critical design objectives. In this role, flexible printed circuit designs are essentially planar wiring assemblies with connectors soldered only to their terminations. More recently, however, this familiar role has been expanded to include multilayer rigid-flex and so-called chip-on-flex (COF) assemblies wherein active and passive devices are attached to the body of the circuit by soldering or thermocompression bonding methods, just as they are in rigid printed circuit assemblies. In this new design context, flexible printed circuits are exposed to more rigorous fabrication and assembly requirements, most notably multiple and extended exposures to temperatures in the 180 to 250.degree. C. range.
Currently, most flexible printed circuits are fabricated from laminates produced by adhesively-bonding preformed copper foil to polyimide or polyester film on either a sheet or roll form basis. Despite their widespread use, these conventional laminates have well-known adhesive-related property limitations which make them particularly unsatisfactory for fine line multilayer and COF designs: poor dimensional stability after etching; elevated levels of retained moisture; high z-axis coefficient of thermal expansion (CTE) values; and excessive thickness. Moreover, due to the fact that the standard Institute for Printed Circuits (IPC) test for thermal stress resistance, IPC-TM-650, Method 2.4.9, method F, is still conducted at 150.degree. C., designers utilizing flexible circuits for the first time may be unaware that the bond strength of adhesive-based laminates, typically 8-9 lbs/in. after thermal cycling at 150.degree. C., deteriorates by more than 50% when cycling is conducted at 180.degree. C. (a common laminating temperature for multilayer constructions) and falls to essentially zero at cycling temperatures above 200.degree. C. (the region of reflow soldering and thermocompression bonding).
These limitations have stimulated interest in a new family of flexible circuit substrate materials based on adhesiveless constructions. In one form, polyimide resin is cast onto a web of copper foil and heat-cured to form a flexible, single-sided metal-dielectric composite; this method, however, is not well-suited to the production of double-sided constructions, an important product category. In another form, polyimide film is directly metalized by either chemical deposition methods (U.S. Pat. Nos. 4,806,395, 4,725,504, 4,868,071) or vacuum deposition methods to produce single- or double-sided constructions. Bare copper itself, however, is not directly bonded to the polymeric film substrate in these constructions because it is well-known that, while reasonably high initial peel strength values of 6-7 lbs/in. can be achieved, the copper-polymer interface in directly-bonded constructions fails catastrophically (delaminates) when exposed to elevated temperature. This phenomenon is generally attributed to the propensity of copper to combine with oxygen or water driven from the film core during the heating process to form copper oxide, a structurally weak and non-passivating interface. Double-sided constructions are especially prone to failure by this mechanism because, as it is converted to a vapor phase, moisture retained in the film core has no means of escape other than via the metal-polymer interfaces. It has also been determined in polyimide-based constructions that the cohesive strength of the polymeric film surface is catalytically degraded by the diffusion of copper into the polymer.
Consequently, in conventional practice, metals such as chromium or nickel or their alloys, which form strong, self-passivating oxides and readily bond to copper, have been employed in film-based adhesiveless substrate materials to serve as a barrier to both the transport of oxygen and the diffusion of copper. Compared to directly-deposited copper, suitable thicknesses of these barrier layer metals do improve the interfacial bond strength retained after thermal exposure but, even so, commercially available adhesiveless substrate materials are not entirely satisfactory in this regard either. Virtually all of these materials exhibit substantial--typically 40% or more--loss of initial bond strength even after thermal cycling at 150.degree. C., a fact that is reflected in IPC-FC-241/18, the acceptability standard for materials of this kind. One explanation for this phenomenon may be that these barrier metals form oxides that are stronger than copper oxide, but only in a relative sense. In the case of materials made by sputtering methods, however, a contributing factor may very well be the industry practice of exposing the polymeric film surface to a so-called plasma etching process prior to the deposition of the barrier layer metal. This process, which is typically performed in an argon-oxygen plasma, is generally considered to enhance barrier metal-polymer adhesion by cleaning the film surface to enhance mechanical adhesion and enriching its oxygen content to promote chemical bonding. Although the latter effect may be of some benefit, it is well-known that argon-oxygen plasmas are essentially ablative in nature and, as such, create relatively smooth, as opposed to roughened, microprofiles which do not materially improve mechanical adhesion. In this regard, it has been found by Ishii, M. et al (Proceedings of the Printed Circuit World Convention VI, San Francisco, Calif., May 11-14, 1993) and others (U.S. Pat. Nos. 4,337,279, 4,382,101, 4,597,828, and 5,413,687) that nitrogen-containing plasmas are more effective.
In addition to being limited with respect to retention of bond strength after thermal cycling, commercially available adhesiveless substrate materials employ barrier layer metals that make the use of these materials problematic with respect to the industry's circuit etching and plating practices. Chromium, for example, cannot be removed by any of the acid or alkaline etchants commonly used in printed circuit operations to remove the copper from the spaces between the trace patterns; removal of the chromium barrier layer also presents a waste disposal problem. Nickel or nickel alloy barrier layers represent an improvement of sorts in that they can be removed in one step with commonly-used acid etchants but, when the overlying copper is removed with any of the alkaline or so-called ammoniacal etchants that predominate in current industry practice, a separate etching step to remove the nickel or nickel alloy is required. It has also been observed by Bergstresser, T. R., et al (Proceedings of Fourth Intl. Conference on Flex Circuits [Flexcon 97], Sunnyvale, Calif., Sep. 22-24, 1997) that when thin layers (less than 200 Angstroms) of nickel or nickel alloy barrier metals are exposed to cyanide gold plating solutions, they are preferentially dissolved. This phenomenon, which leads to undercutting of the copper traces and consequent loss of metal-polymer adhesion, is especially problematic in the fabrication of very fine line designs with trace/space geometries less than 4 mils. Titanium is another well-known barrier layer metal which has been used in semiconductor manufacturing processes to enhance the adhesion of copper deposited onto a spun-on layer of liquid polyimide. However, titanium metal has not been used as a barrier layer in adhesiveless flexible circuit substrate constructions because its removal requires a second etching process that involves special chemistry.
As a means of addresssing the etchability issue, it has been proposed in U.S. Pat. No. 5,137,791 to form an adhesiveless polymer film-metal composite without the benefit of a conventional metal barrier layer by first using an oxygen plasma containing multiple metal electrodes to simultaneously treat the film surface and deposit an extremely thin, non-continuous layer of a metal oxide; a thicker second metal layer such as copper is then deposited over the first layer. Although initial peel strength values greater than 6 lbs/in were reported for polyimide film-based constructions of this kind, no thermal cycling data was provided; it has been found that when a composite material made by this process is subjected to thermal cycling, the peel strength of the metal-polymer bond rapidly degrades. U.S. Pat. No. 5,372,848 proposes to provide for single-stage alkaline etchability by the deposition of a copper nitride barrier layer directly onto an untreated polyimide film surface. Although composites made by this process are alkaline-etchable in one step, it has been found that their initially high adhesion values deteriorate significantly when exposed to elevated temperature. It has been proposed by Weber, A. et al (Journal of the Electrochemical Society, Vol. 144, No.3, March 1997) to use chemical vapor deposition methods to deposit onto polymer-coated silicon wafers a thin titanium nitride barrier layer sufficiently conductive to permit direct electroplating of copper. It has been found that thin TiN barrier layers formed by sputtering methods are too resistive to accomplish direct electroplating of copper and that even sputter-deposited copper does not form a strong bond with TiN because of its stoichiometry.
Thus, efforts to improve the initial/retained peel strength values and chemical processing properties of film-based adhesiveless substrate materials have taken many forms but no completely satisfactory result for flexible printed circuit applications has emerged, nor has the prior art taken the specific form of the novel materials system proposed in this invention.